The present invention relates generally to a compact zoom lens system suitable for lens shutter cameras, etc., and more particularly to a three-unit or positive/positive/negative-unit, fast zoom lens system which has a zoom ratio of ca. 2 and a small F-number.
In recent years, lens shutter cameras with built-in zoom lenses have become widely popular, and numerous zoom lenses suitable for lens shutter cameras, etc. have been proposed until now.
For instance, the zoom lenses proposed in Japanese Provisional Patent Publication Nos. 56-128911 and 57-201213 each comprise two or positive and negative lens units. However, when it is intended to correct lens performance well with a two-unit zoom lens, some difficulty is encountered in having a zoom ratio of 2 or more. Especially when it is intended to obtain a small F-number of ca. 3.5 for instance at the wide position, the achievable zoom ratio is of the order of at most 1.5.
For this reason, a number of three-unit zoom lens systems have been proposed as zoom lenses having a zoom ratio of 2 or more. According to Japanese Provisional Patent Publication Nos. Sho. 63-153351 and Hei. 1-252916, zoom lenses having high zoom ratios are achieved by constructing the optical systems of three or positive/positive/negative lens units.
In the three- or positive/positive/negative-unit zoom lenses proposed so far in the art, however, the optical performance is maintained at the expense of the F-number. When the focal length of the total system at the tele position is of the order of 35 mm, the F-number is about 4.5, but this F-number is much more increased in association with zooming to the tele position. In addition, when the focal length of the total system is of the order of 80 mm, the F-number will assume a value of 6 or more.
When it is intended to design a fast zoom lens having a small F-number ith the lens arrangement proposed so far in the art it is imperatively required to diminish the refracting power of each lens. This gives rise to an excessive increase in the size of the lens system, failing to provide a zoom lens suitable for use with a compact camera.